Recent advances in spectrometer designs, such as those described in U.S. Utility patent application Ser. No. 10/749,363, particularly in the long wavelength infrared (LWIR) spectrum, have resulted in unprecedented levels of compactness (see FIG. 1). As a result, it has become increasingly more feasible to incorporate the spectrometer itself into the Dewar assembly, thereby providing several advantages to the optical system. These advantages include, but are not limited to, a higher cold stop efficiency (virtually 100%) due to the optical stop being physically located within the cryogenic Dewar environment, a reduction in stray light resulting from the significantly smaller, slit proximate Dewar entrance window, and a significant reduction in background radiation from the cold optics of the spectrometer.
Because spectrometers are inherently rotationally asymmetric, they require several additional forms of alignment over conventional symmetric optical systems in order to operate properly. These typically include the rotational alignment of the sit aperture, dispersing element, and detecting element relative to one another, as well as the standard focus adjustment. Other alignments, such as the horizontal and vertical translational alignment of the slit aperture with respect to the detector element may also be necessary to meet system performance objectives. For optical systems that are located within a Dewar environment, this task typically requires first cooling the system down in order to take data measurements, then warming the system back up in order to make alignment adjustments. Because of the expansion and contraction of materials over such large temperature changes, a precise adjustment cannot be made reliably, and this process must typically be performed over several iterations, taking a great deal of time.
A rotational misalignment between the slit aperture, dispersing element, and detector element in a spectrometer can result in numerous spectral and spatial inaccuracies, and a significantly high degree of rotational alignment is typically required in order to provide accurate spectral signature information to data processing algorithms. This requirement is often on the order of one-tenth or one-twentieth of a pixel maximum displacement of a line image at the edge of the detector, which for a 512 pixel width detector array corresponds to approximately 0.2 milliradians of rotation. It is desirable to provide an angular resolution of at least half this value, or 0.1 milliradians, and for a spectrometer housing with a diameter of 20 millimeters that is driven by the translational motion of an actuator located at the circumference of the housing, the resulting required translational resolution of the actuator is approximately 1 micron.
Since a spectrometer is an imaging device, it is typically necessary to provide a method of focus alignment such that the line image presented at the slit aperture is properly focused at the detecting element. This requirement depends very strongly on the optical speed of the system, and can be determined by the axial translation necessary to generate a given change in spot size at the detector. This change is often on the order of 10 percent, which for a diffraction-limited F1.5 spectrometer system corresponds to approximately 10 microns of translation.
There are also a variety of other optical and sensor applications that require precise alignment. There is also a need for miniaturized and sensor optical applications.